This invention relates generally to mass transfer columns and, more particularly, to an apparatus and method for supporting trays within such columns.
Mass transfer columns, including heat exchange columns, typically include an upright shell and a plurality of horizontally disposed trays that are positioned within the shell and are used to facilitate mass or heat transfer between fluid streams flowing within the column. The fluid streams are normally one or more downwardly flowing liquid streams and one or more ascending vapor streams, although other combinations of fluid streams are possible. Each tray includes a plurality of vapor passages that allow the vapor stream to ascend through the tray for interaction with the liquid stream flowing across an upper surface of the tray. A plurality of such trays are normally supported in vertically spaced relationship by support rings that are welded to the inner surface of the shell and underlie the outer peripheral edge portion of the trays.
In the type of vapor-liquid contact trays disclosed in U.S. Pat. No. 3,045,989 to Kittel, a center downcomer and an outer annular downcomer are provided on alternate trays. As a result of this downcomer arrangement, liquid flows radially or along a spiral flow path from the center to the perimeter of alternating trays known as centrifugal contact trays and then from the perimeter to the center of the remaining trays known as return contact trays. Conventionally, a center support pipe that extends longitudinally along the center upright axis of the column carries support rings that are used to provide center support for the centrifugal trays. The use of this center support pipe, however, reduces tray capacity because it occupies a portion of the center downcomer on the return trays as well as a portion of the tray deck on the centrifugal trays. Installation of the center support pipe can also be problematic because it is often too long to be inserted in one piece through the entry manholes. As a result, the center support pipe must be divided into a number of shorter length segments that can be carried into the column and reassembled by welding the segments together end to end. Further installation delays result from the need to position support beams within the column to support the lower end of the center support pipe and from the need to position and weld tray support rings on the center pipe. The center downcomer must also be assembled around the center support pipe, causing further delays during installation of the trays.
A need has thus developed for a way to support centrifugal and return contact trays of the type described above without using a center support pipe.